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What are some drawbacks to having a wingtip propeller on an aircraft?
I was reading about aircraft concepts that involved wingtip propeller design and was wondering what the drawbacks of such a technology would be. I could not access the full paper but came across this: https://arc.aiaa.org/doi/pdf/10.2514/3.44076
For me, one of my concerns is that if the propeller fails, feathering would be very difficult on the wingtip prop. I am also trying to decide on optimal placement for aircraft propellers in general. Would the drawbacks from having a wingtip propeller outweigh the benefits in high-lift generation and drag reduction?

"if the propeller fails, feathering would be very difficult on the wingtip prop" Why, and how does the feathering mechanism differ based on the spanwise mounting location? This does not compute for me.

Loosely related: https://aviation.stackexchange.com/q/13382/2407

Could you perhaps link to something that clearly shows what you mean? When I google wingtip propeller the first hit is this actual question, and there seems to be conflicting results from a google image search.

If you were clever enough with tip-jets, you could solve some of these drawbacks while introducing many, many others.

@AEhere Sorry, I realize that what I meant was that failure on the wingtip propeller would cause a moment on the aircraft and stability would be difficult. I am a bit new to aircraft desgin and English is not my first language so my apologies for confusion.

@pipe https://www.researchgate.net/publication/329036195_Wingtip-Mounted_Propellers_Aerodynamic_Analysis_of_Interaction_Effects_and_Comparison_with_Conventional_Layout/figures this is what I am referring to

Feathering isn't really an issue, if you can feather a prop on an engine further inboard you can do it on the tip too. There are 3 major drawbacks that come to mind:

The wing structure has to be stronger: engines are heavy, the further out they are the beefier the structure has to be to hold them. Stronger wings mean more weight and possibly a thicker cross section. Neither are good traits in a wing
Decreased roll rate: the farther the engines are the greater the moment arm and the slower your roll rate will be. Think about skaters spinning around, the farther their arms from their body the slower they spin. The same principle is at work here, so you need bigger ailerons to give you maneuverability, so more weight, cost and complexity
Safety in a single engine failure scenario: engines on the wingtips will cause more yaw in a single engine failure than engines further in board, so you'll need a bigger rudder to counteract it. A bigger rudder means more weight and cost, and there are also limits - eventually you will get to the point you can't counteract the force effectively and an engine failure will cause a loss of control. Yaw onset will be faster as well, giving a pilot less time to react, and there's nothing you can do about that; a bigger rudder doesn't help with reaction time. Mechanical cross-connections could be used to share power across the wing in the case of a single engine failure, like the V-22 Osprey, Chinook helicopter, however these increase weight, cost and complexity. Also, these systems aren't perfect, a single engine failure is still a possibility

The Flying Flapjack series of experimental aircraft had the engines buried in the wing, at roughly similar locations to a conventional twin.  They also used a shaft cross-connect with overrun clutches (much later adopted for the test series leading up to the Osprey tilt-rotor) so that either engine failing would allow normal operation on the remaining engine with balanced thrust.

I'm not saying it can't be done @ZeissIkon, just that there are design considerations.  I am familiar with cross connects, failure of cross-connects in the Osprey and its predecessor. has led to at least one fatal crash.

Not sure about the wings. Engines are heavy, but they are not held up but the fuselage. Instead, the wings hold up both the fuselage and the engines. By moving the engines away from the fuselage, the load is spread better over the wing, allowing for lighter wings (except on the ground, but dynamic loads in flight are the design constraint. These can be far more than 1g)

@MSalters Erm, yeah, but doesn't a longer lever always exert more force when the same mass is applied? The same applies of course to any dynamic force added - like vibration.

@Raffzahn: The mistake is treating the wing as a lever. It is, on the ground, where the wheels hold up the whole plane. But the larger loads are the dynamic loads in the air, where the wing is generating the lift. that means the wing is no longer a lever transferring the weight of the engine to the fuselage. Instead, both the engine and the fuselage are now weights hanging off the wing.

@MSalters It it still a lever just because the load distribution on the lever changes, doesn't mean itself is changed. Every engine vibration has now the whole wing length as a lever against the fuselage. And at the same time, the wing needs to be strengthened all the way to the tip to hold the engines weight in flight. After all, lift is (hopefully) distributed all over the wing, while the engines weight is still on the outer end - the one wich has with conventional mounts the least strength. So no matter how it's turned, as farther out the engines are, as heavier the wing has to get.

@Raffzahn: Sorry, but you seem to have a mental model of how those vibrations are transmitted that I cannot match with my understanding of physics. And yes, the wingload distribution changes of course. That's why the wings can be lighter. In the current situation, the largest part of the wing is outboard of the engines, which means very high stresses in the wingspar just outside the engine.

@Raffzahn Imagine a little model airplane. If it's on the ground, the wheels on the fuselage are supporting it. So support your little model from underneath the fuselage with your hands. In this case, the weight of wingtip engines does stress the wing root more. BUT in the air, the wings are supporting the plane. So now hold the plane from underneath but with your hands in the middle of each wing. Now tell me, is it harder on the wing root if the engine mass is on the fuselage? Or on the tips? Obviously its harder on the wing root if it is on the fuselage in this case.

Another non-flying attribute that having a wingtip-mounted propeller can drive is landing gear length. Since there are minimum clearance distances for propeller tip to ground during taxiing, and the wings may sag or dip during a turn while taxiing, you may end up having to change your gear length (which can cause other issues in turn).

Moving the thrust (and additional weight) to the wingtips creates more drawbacks than benefits.
Yaw - because of the increased moment of inertia (compared to having the engines be closer to the fuselage - the center line of the mass), it would be harder to initiate yaw changes as well as harder to stop or reverse them.  Left/right thrust differentials could be used, and that would certainly increase yaw change rate, but then you have to consider the time cost of changing the force of each engine quickly.  And it becomes a very serious problem if you have a failure on one side, leaving you with only one wingtip generating all of the thrust.  Depending on the geometry of the aircraft and the size of the vertical stabilizer, it might not even be possible to counter the yaw force generated by the one engine producing enough thrust to keep the aircraft flying.
Roll - Similar to the yaw problem, the roll rate would be reduced the further the weights were moved away from the center line.
The V-22 Osprey is an example of this design.  The wings are kept short to minimize the increase to moment of inertia, but the operational requirements of the vehicle (VTOL) required it to have large propellers (rotors), so the wings had to be long enough to keep the prop tips from hitting the fuselage.
Additionally, vibration and external (turbulence) effects on the wing structures would have to be considered.  Even in normal operating conditions, the wings would be subject to increased vibrations that could create structure failures in some complex compound wave situations.  Aircraft designers already deal with this and model these scenarios, but the complexity increases (I suspect exponentially) as the vibrational force is moved further toward the wingtip.

Why would the vibration problem worsen if you move the engines away from the fuselage? The fuselage isn't dampening the vibrations. Instead, it's where the most vulnerable parts are located. Keeping vibration sources away from it is a good thing.

@MSalters Distance (in this case from wingtip to fuselage) determines the maximum wavelength that the medium (the wing) can support.  Since the engine can be seen as one force that can generate vibration, and the fuselage can be seen as the anchor or resisting force, a longer wing means longer waves (more flex), plus more complex harmonics.

The Navy V-173 "Flying Flapjack" was an example of this design, flying in the mid 1940s.
Development of electric technology has brought this concept to light again for wingtip propeller mitigation of vortex drag so we again return to the question of efficiency.
Aerodynamicly, this is a question of drag for a given weight at a given speed.  Wingtip motors would require additional strengthening of the wings and larger roll control surfaces for the same performance, adding weight to the design.  Adding weight adds to drag, canceling any savings in drag from tip vortex reduction.
Propulsive efficiency must also be considered.  Breaking 2 propulsion units down to 4 or 8 also results in a loss of efficiency. Indeed, the trend is opposite, as seen with the Boeing 777.
In summary, keeping 2 propulsion units at the wing tips (like the Flapjack or the Osprey V-22) will result in a lower aspect and/or heavier air craft, while splitting propulsion units increases fuel consumption per unit thrust.
Where are the gains? Somewhere between airplane and helicopter, which is where the Osprey is for military purposes.  Practical, cost effective commercial applications remain to be seen.  Lower cost fuel could be a driver of development.

To me a showstopper disadvantage of wingtip propellers is that they are potentially disastrous in crosswind or landings, or landings with significant turbulence. Any pilot who has flown long enough has had landings where wingtips came quite close to the ground. In fact in a cross wind landing you should have your wing on the wind side a bit lower than the other wing, which will be in partial shadow from the fuselage.
With wingtip propellers now you have less clearance for surprise gusts but more importantly the consequences are far worse. You could damage a prop and suddenly have asymmetrical thrust at the worst possible time.
A second disadvantage is collisions on the ground. Wingtip scrapes are relatively common in all sizes of aircraft used by all types of operators. They are nearly always inconsequential. With a wingtip propeller these incidents would cause a lot more damage to both craft and are more likely to cause injury due to flying debris.
And of course as others have mentioned, loss of a motor implies that the center of thrust will be very far from the center of mass and aerodynamic center of the aircraft. On takeoff particularly this is not what you want. It is hard enough to manage with regular twins, which have the engines as far inboard as possible.
It's just a bad idea.

Sophisticated flight control systems of the type needed for fully autonomous flight can handle crosswind landings while keeping the wings exactly level, via superhumanly precise implementation of the kick-out-the crab method of landing.  Fully castoring landing gear allowing landing in a "crabbed" orientation may be an even better solution-- any drawbacks of such a configuration may be overcome by the superhuman precision of modern autonomous  flight control systems.

Wrapup of smaller things:
This could affect the minimum runway width required.  Wings will pass over grass okay, but props/engines could risk blowing dirt/dust/plants about and inhaling them causing FOD.   Any obstruction beside the runway could have consequences.
Slight increase in fire risk from any sparks from the engine because the spark might drop into shelter rather than dropping onto the hard tarmac of the runway.
Increased risk to first responders in the event of an incident/accident because the moving parts may force crash tenders to stand back a bit further slowing the quench time of fire.
Engines over grass/soft ground could make landings and take offs slightly quieter as a benefit.  I'm unsure if the passengers would find it quieter or louder.

First of all, not all but most of the answers you have received are based in psychological inertia, loss aversion, confirmation bias, availability cascade and shoehorning of scenarios, and so they are subjective, poor and based in government sponsored petrochemical industry ICE piston or jet engine obsolete technology, that has hijacked aviation technology for so long with inefficient aerodynamics and propulsion systems, that in this case has limitation in propulsion placement because of the whirl created by all those engines's moving pieces and other factors (the unlikes of this post prove that fundamentalism that has nothing to do with aviation, technology or science, but with fake technology advocacy that avoids and tries to suppress any challenge to the technological status quo. as today there have been 3 or 4 unlikes that have countered the 2 or 3 likes of this answer and keep this answer as negative. this type of behaviour is a plage in several pseudo technological aviation or boat design forums created by biased people hoping for advertising revenue or even wiki pages, which i do not recommend to enter).
With other types of propulsion as battery electric, hydrogen electric, solar electric, solar stirling or isothermal compressed air ICA propulsion ( https://www.reddit.com/r/AerospaceEngineering/comments/pkmdz6/stable_flying_wing_design_setup_propelled_by/ ) there are no barriers to propulsion placement and so there can be several configurations as distributed electric propulsion DEP with boundary layer suction or ingestion and wing tip propellers as nasa x-57 (https://www.nasa.gov/aeroresearch/X-57/technical/index.html) and other publications (https://www.researchgate.net/publication/329036195_Wingtip-Mounted_Propellers_Aerodynamic_Analysis_of_Interaction_Effects_and_Comparison_with_Conventional_Layout).
Rather than a simplification or eliminating discourse of centre propeller vs wing tip propellers, they can be a complement for the main aircraft propulsion and can play a role in swept angle tailless aircrafts by moving part of the propulsion further aft (tailless aircrafts normally use push propulsion for that effect), which are far more efficient that today's petrol aircrafts with cylindrical fuselage and short aspect ratio wings.
Wing tip propellers can increase lift that is very important for STOL development, thrust and reduce induced drag by rotating counterwise to the wingtip vortex (nasa x-57) and they can increase stability of the aircraft by placing them off-axis (NEW EDIT: this is the real publication: Wingtip-mounted propellers installed at a nonzero yaw angle 2021 Thesis of Yannick Chance https://repository.tudelft.nl/islandora/object/uuid:7aceebd5-4221-41f0-8657-3b61b152cad0) what can open the door to fly-by-wire 3d gyrating or 2d rotating wing tips propellers as stability assistance during adverse weather conditions instead spoilers or ailerons, what can increase also roll rate and manoeuvrability.
Also in the case of general aviation aircrafts, by being placed in the tips the propellers create less parasitical drag compared to a pull centre propeller that creates additional turbulences and parasitical drag trough all the fuselage, a drag rate that increases exponentially with the speed.
As propellers and propeller arrays have several uses as take-off thrust, climbing or soaring thrust, cruising thrust or gliding assistance thrust, in the end wing tips propellers can help to reduce drag of sailplanes at small rpm or reduce drag during cruising in different aircrafts with additional thrust, besides increasing the mentioned directional stability of aircrafts and improve STOL performance.
there are several trade-offs of wing-tip propulsion, and it will vary by the type of propeller or engine and also from aircrafts with only wing tip propulsion to with aircrafts that use additional propulsion besides wing tip propulsion, and also from tailless aircrafts to tailplanes, and push or pull configurations, and cruising speed as well:
-As with DEP there is wing and whirl flutter and so the need of additional structural reinforcement, however the weight and whirl of electric motors are a small percentage of a combustion engine and even less in the case of pistonless rotary air engines, and also flutter can be further reduced by using a small or partial percentage of the total aircraft thrust in the wing tips.
-Width increase and propeller crash on landing can be a problem by using fixed propellers, however as today there are propellers that can be folded so this issue can be totally avoided.
-single engine failure will be a problem in aircrafts that only use wing tip propulsion, and compared to twin engine aircrafts because it still will be an advantage compared to an engine failure in general aviation single ICE engine aircrafts.
-yaw and roll performance in the case of aircrafts using only wing tip fixed axis propulsion, however an aircraft that uses gyrating or rotating propellers in the wing tips will have better yaw and roll performance than today's single or twin propeller tailplanes that depend from spoilers, alieron and rudder besides having a greater stability control, and for tailless aircrafts this means a gamechanging technology that is currently in development or to be developed.

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.

I added additional information with the actual publication links, including a post i made in aerospace reddit forum and  ended ditching wing tip propellers for take off thrust assistance, but i could use small foldable propellers with pneumatic actuators only for stability control in demanding scenarios (there's a video in youtube with a short and partial analysis of wingtip propellers https://www.youtube.com/watch?v=Vi9FqIAG0Rg). Besides the bias i have noted in this thread there have been valid points that help to understand wing tip propeller technology's trade-ins and trade-offs

while wing tip propellers add complexity compared to winglets, they can increase wing lift and they can be a valid choice for some uses and by moving the axis of the propellers they can increase stability control of the aircraft compared to spoilers or ailerons, mostly in high aspect ratio sailplanes, flying wings and blended wind body aircrafts.

again this question is about wing tip propulsion and the question is wrongly placed because it must have asked what are the drawbacks and trade-ins of this technology instead of asking what's wrong with wing tip propellers to not being included in the aviation status quo, and so this negative question has open the door for compulsive smearing of this technology. after reading this thread looks like wing tip propellers are worthless and have to be thrown to the bin.

i extended the trade-offs information

I am locking this answer to stop the continual minor updates which has the effect of bumping it back up the homepage.


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Can I use an aviation radio on the ground in an emergency?
Suppose I'm doing some plane spotting, and I see a dangerous situation that I think needs to be brought to the attention of a pilot or ATC (such as a tire falling off an airplane, an engine fire, etc.). I have a handheld aviation radio tuned, so I know that nobody's reported it. Would it be permissible for me to use that radio to call it in? How would I identify myself since I'm not in a plane and therefore have no tail number? Would I have to identify myself at all?

One problem you face is that you do not have a radio license that permits you to transmit on that band.  In many countries, these license are explicitly granted to pilots.  In the U.S., I believe radio licenses are usually implicitly granted along with the pilot's license.  But you have to have one (legally).

Pilots operating domestically in the USA are exempt from FCC permitting/licensing, it is an exemption not an implicit license. US pilots must obtain an FCC radio operators permit before flying internationally. The aircraft itself may have some station license requirements, but I am not an owner so never looked into it.

You can, and should, use any means necessary to render assistance that might be needed in an emergency and not worry about lack of an FCC ground station license stopping you. However, good judgement is needed to discern the severity, level of help needed or wanted, and the value your transmission might provide.
For example, I would be extremely reluctant to key the mic on a heavy Airbus at a major airport on tower freq if they are trailing a little smoke because it's likely they have cockpit indications already, and you'd probably get a whole lot of unwanted attention afterwards.  But if I were at a small rural non-towered field on Unicom I wouldn't hesitate for a second to transmit "Bonanza on final, your right main gear is NOT down!"
I can't speak as to how the FCC might view your actions, but knowing that the CFRs allow a PIC to deviate from any regulation to the extent needed to meet the emergency, and that many states have Good Samaritan laws to protect people from being persecuted for helping others in distress, I would rather have a clear conscience and take action in good faith than cower in the face of potential repercussions.
Oh, to answer your last question, (and to mitigate those FCC repercussions...) I would probably just NOT identify myself.  ;)

The FCC would actually not smile on this at all.  (I’m also a lm amateur radio and commercial radio operator licensee.)  There was a case some time ago where a ham radio operator used his radio to make a bona fire emergency call on a frequency he was not licensed for.  He was revoked by the FCC and was ultimately convicted.  The court acknowledged that it was a bona fide emergency, but held that Good Samaritan doctrine did not apply.  No jail time, but his minor fine and license revocation were upheld.

@Max - That's your government in action...  Good to know, and a good reason to not identify yourself.  Personally I would give up a stupid license to save a life.  (especially if I was unlicensed to begin with!)

I agree with your original comment, one approach is to make the call and then not identify yourself.  Reminds me of that commercial, “This is Peggy.”  Here’s part of what makes this a hard call…  If I see a plane fall out of the sky, then I know this is an emergency and I know I have a moral justification, even if my license is at risk.  But a tire bouncing down a runway is a different calculus for a non pilot.  “WTH was that, WTH does that even mean, WTH did I just see?” is a harder starting position for taking a risky action.

@Max do you have a source for the "revoked after an emergency" story?

@Max given 47 CFR § 97.403 "Safety of life and protection of property.
No provision of these rules prevents the use by an amateur station of any means of radiocommunication at its disposal to provide essential communication needs in connection with the immediate safety of human life and immediate protection of property when normal communication systems are not available." you need a good source for that.

@JonCuster, That's a great reference.  You have first dibs for finding it, but if you don't post an answer yourself would you mind if I incorporated it into my answer?

@MichaelHall - feel free. I would point out that that is under the amateur rules, not aviation rules. Amateur is part 97, aviation is part 87. I see "§ 87.397 Emergency operations.
(a) The licensee of any land station in the Aviation services, during a local emergency involving the safety of life and property may communicate in a manner other than that specified in the license" which is quite different - the OP would need to be a licensee even in an emergency?

I can’t find a specific source, it would have been 1991 or 92, because I remember the meeting where we discussed the implications of this.  It seemed like a no-brainer to me.  This was an amateur whose 2m handheld was capable of transmitting on public safety bands in the 150MHz range.  He called a police dispatcher directly as a result of an accident.  His club repeater also had autopatch, and he was within coverage.  It was held that he had another means.  97.405 relaxes the “other means” requirement if the station itself is “in distress”…  (cont)

But 97.403, reporting on behalf of the public safety or safety of others, has an elevated standard of “normal communication systems are not available.”  It seemed insane to us that he was cited, but it was noteworthy enough that the ARRL sent a communication out about it and we discussed that communication at our club meeting, that’s how I remember the timeframe.  There are many, many things in life that you can do 100 times…  90 times no one cares, 9 times someone escalates it but it ultimately blows over…  and one time in 100 the snake bites.

Given all the comments and other references maybe it's better to leave as-is.  As a general rule regulatory bodies have jurisdiction over licensees, and enforcement actions are limited to revocation of licenses for failing to adhere to the terms of the "contract".  Which is motivation enough for those interested in keeping their license.  However, I think it would require a higher legal standard for an agency to pursue criminal action against an unlicensed individual, and they would probably have to prove negligence or that harm was done.  But that's a better question for Law SE.

source: Stack Exchange

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sort: votes


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Should I call ahead to land at a Class B airport?
I'm a private pilot, not instrument rated. I want to land at a Class B airport (New Orleans International KMSY, if that makes a difference), just for the experience of having done so. According to my research, most pilots recommend calling ahead and finding out when they're not going to be busy.
Is that good advice? How far ahead should I call? What number should I call?

Might want to ask about landing fees...

I'm voting to close this as it's clearly opinion based, as is how far ahead should a call be made. It's not as if you lost anything by making a call.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Which FCC license do I need to operate an aviation radio on the ground?
I'm in the process of buying a handheld radio as a backup for when I go flying. According to the owner's manual, I'd need a ground station license to operate it when I'm not actually in an airplane, and to fill out FCC Form 406. But, I can't find any information on a "Form 406" on the FAA's website, and the online application has five different options that seem to be aviation-related ("AA - Aviation Auxiliary Group", "AC - Aircraft", "AF - Aeronautical and Fixed", "CA - Commercial Air-ground Radiotelephone", and "CG - General Aviation Air-ground Radiotelephone"), and I can't figure out which one is applicable.
So, if I wanted to use my shiny new aviation radio on the ground, legally speaking, what license would I need?
(I already have a Restricted Radiotelephone license, if that helps.)

Why do you need to use it outside of an aircraft? Just to chat? Or just to listen?

@RonBeyer I actually can't think of a reason right now why I might need to do so (other than maybe a one-time test). I just like having the option.

@RonBeyer - I’ve always wondered if a CFI at a non-towered field would find it useful to be able to use a handheld while their student solos. I know that parachute drop zones have airband ground radios. And, of course, FBOs have them. What about an individual? Just curious.

Listening probably doesn't require a license, the problem is if you want to talk. FBO's have unicom and are probably allowed under some FAA/FCC rule without the license. If you want to test the radio, sit in the airplane, it's a better test anyway. If you really want the license, I think you need the "CG" version.

There is an FCC form 601, which says it replaces form 406 (and many more)

Part 87, more properly 47 CFR § 87 defines many different types of stations that use the FCC's "Aviation Services" (the legal use of radios on aviation bands in the US).  See 47 CFR § 87.5 - Definitions.  Here are some examples:

Aeronautical advisory station (unicom). An aeronautical station used
for advisory and civil defense communications primarily with private
aircraft stations.
Aeronautical enroute station. An aeronautical station which
communicates with aircraft stations in flight status or with other
aeronautical enroute stations.
Aeronautical multicom station. An aeronautical station used to provide
communications to conduct the activities being performed by, or
directed from, private aircraft.
Aeronautical search and rescue station. An aeronautical station for
communication with aircraft and other aeronautical search and rescue
stations pertaining to search and rescue activities with aircraft.
[I'll skip a few...]
Aircraft station. A mobile station in the aeronautical mobile service
other than a survival craft station, located on board an aircraft.
[Skipping a few more...]
Aviation support station. An aeronautical station used to coordinate
aviation services with aircraft and to communicate with aircraft engaged in
unique or specialized activities. (See subpart K)
[There are several more.]

So before you apply, you'll need to know exactly what kind of station license you're applying for.  The FCC has a nice web page that explains some of the various types of ground stations for the aviation services bands that the law recognizes.
If "aviation support station" caught your eye, that type of ground station is explained more fully in section 87.319.  That section is quite specific, and "aviation support station" isn't just a miscellaneous catch-all category.  By the way, it looks as though FBOs use aviation support station licenses.
Section 87.18 says, "An aircraft station is licensed by rule and does not need an individual license issued by the FCC if the aircraft station is not required by statute, treaty, or agreement to which the United States is signatory to carry a radio, and the aircraft station does not make international flights or communications."
So you use your shiny new radio in an aircraft without a license, as long as you don't plan to cross any borders.  However, if you don't have a station license and you want to test your radio, then you should probably be testing it from inside an aircraft.  (Take my advice with a healthy dose of skepticism, because I'm not a lawyer.)
On the other hand, Section 87.47 says, "A person may apply for a portable aircraft radio station license if the need exists to operate the same station on more than one U.S. aircraft."  So if you want to use your shiny new radio in more than one aircraft, then you may apply for a portable aircraft radio station license.  However, there really is no need for a portable aircraft radio station license for a pilot who wants to use a radio in more than one aircraft and intends to communicate only inside the US, because the license is implied in each aircraft.  (Hat tip to @AJHenderson.)
The way to apply for any of the Aviation Services licenses (besides the basic aircraft station license for which no license is required) is to register with the FCC's Universal Licensing System (ULS), and then file a license application electronically there, which will be the equivalent of the old Form 601, which replaced the older form 406.  Depending on how you applied for your restricted radiotelephone license, you may already have the FRN (Federal Registration Number) to use with the ULS.
One commenter wondered about the meaning of the word "station" in the law.  Essentially, a station is a radio.  The terminology dates back to the vacuum tube era, when radios were large and cumbersome, and most radio transmitters were permanently installed in buildings; portable radios capable of transmitting weighed 25 lbs or more, and typically had low output power.  For most radio "services" (the radio broadcast services, the amateur service, etc.), including the aviation services, the license is associated with the station (the transmitter or radio) rather than the operator.

It is worth adding that the chances of actually getting a ground license are pretty slim.  You have to be in one of a very select group of roles to be granted a ground station license.

It's also likely worth highlighting that you don't need to have the portable license for moving between planes domestically as each plane would have it's own implied station.  The main reason I can think of to apply for a portable license would be if you wanted to fly internationally in multiple aircraft and not have to worry about if they had a station filed or not, though this might make radio calls more interesting since you would need to use your callsign and identify your plane (or would you just use the callsign maybe?)

Interesting choice of words in 87.47.  Because “MAY apply if there’s a NEED” is not even close to “MUST apply if there’s a WANT.”  That language makes it 100% optional in any circumstance.  Also, is there any official definition of the word “station”?  Because “handheld portable station” is an oxymoron unless radio = station.  (I think a strong case can be made from plain language definitions that they are not the same thing). Summary: good compelling answer, but I’m as confused as ever...

Thanks @MichaelHall!  I added a paragraph to the answer clarifying what a station is.

I think you're right that the FCC reserves the right to evaluate the degree of need when deciding whether or not to grant a license.

the portable license may well be useful for ground vehicles operated by airports. These usually tune in to the same ground frequencies (and I guess unicom on untowered fields) as the aircraft but aren't aircraft so would probably need a license.

This is the same as when using an aviation radio inside the airplane. The operator of the aviation handheld radio does not need a license to operate the radio within the US.  You only need a license to operate outside of the US.
And it would also be the same Restricted Radiotelephone license you already have.

Welcome to aviation.SE! Do you have a source for this? 47 CFR 87.18 says that aircraft stations don't need a license, but I don't see the same exemption for ground stations.

@Pondlife, are there any FCC references equating portable devices with stations?  Because I think they are different…

@MichaelHall See this: "You may only use your hand-held aircraft VHF radio in your aircraft under the terms of your aircraft license. You are required to have a separate Ground Station license to operate your radio on the ground." The link goes here, it looks like "aviation support station" covers what many people use handhelds for.

@Pondlife, that first quote is a slam dunk answer to the question, but it doesn't appear in the link you provided.  Would you please verify where that came from?

@MichaelHall Sure, it’s on the page I linked but you need to click the Operations ‘tab’ or header to see it.

@Pondlife Aviation support stations are explained in section 87.319, which is quite specific.  One has to be an FBO, a balloon chaser, part of a soaring club, or a pilot trainer, or demonstrate that the license will be used for "promotion of safety of life and property".  (The last bit probably covers safety and fire crews at large airports.)  "Aviation support station" isn't just a miscellaneous category.

@rclocher3 Yes, I agree. The FCC's page says that handheld aviation radios require a ground station license, and it lists various types. To me, the description of aviation support seems like the closest fit to how a GA pilot might use their handheld, but I could be wrong.


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What other advantage(s) does an adjustable prop give you?
From what I understand, the purpose of a variable-pitch propeller (be it manually adjustable, or constant speed) is that it allows the engine to always operate at its most efficient speed, much like the transmission of a car. Is that correct? If so, if an airplane were to accomplish that another way (say, a hybrid gas-electric drive, or an actual transmission with shiftable gears*), would there be any other advantage to having an adjustable propeller?
*No, I don't know why you would want to mount a car transmission on an airplane. It's a hypothetical question. But now I have this picture in my head of a twin-engine plane with two gear shifts in the cockpit and two clutch pedals next to the rudder pedals.

The comparison with a car's gearbox is unfortunate, and leads to misunderstandings...
At the small airspeeds typical of takeoff, the inflow at the rotor plane is small too, and the prop needs a small amount of pitch in order to produce a large amount of thrust at maximum engine revs, the blades working at the angle of attack for best L/D.
But, on cruising flight, there is an important inflow at the plane of the prop disk, that inflow reduces the angle of attack, and –under those conditions– the propeller can deliver enough thrust at maximum engine revs (i.e., with the blades working at the angle of attack for best L/D) only if the pitch is substantially higher than at takeoff...
That's the rationale of the adjustable prop...

It's not just the operating RPM that is pertinent. It's also the pitch of the blades.
If your propeller blades' pitch is optimized for takeoff/climb performance, it's going to be aerodynamically inefficient at high true airspeeds, like taking little tiny footsteps on a treadmill that's going really fast. (It works, but it wastes energy.)
The opposite is true as well. If you have a "cruise" prop on your airplane, it's going to perform poorly on takeoff when the airspeed is slow, like taking large strides on a treadmill that's just barely moving.
If you want to optimize your body mechanics, you change your gait based on the speed you need to go, so you're not wasting energy. If you want optimize a propeller, you change it's pitch based on the speed of the air that's entering it, so you're not wasting energy.

essentially using the blade at too high(stalled) or too low angle of attack.

Next to the aerodynamic advantages (both engine and propeller can run at their optimum speed for best efficiency) already mentioned in the other answers, there is an important safety advantage: a variable pitch propeller can be feathered such that its drag after an engine failure is minimized. This is critical for many multi-engine aircraft in order to remain controllable after an engine failure.

On many variable pitch propellers, the blade pitch can be increased to the point that the chord line of the blade is approximately parallel to the on-coming airflow. This process is referred to as feathering.
The inflight feathering of the propeller, on an engine that has failed or has been intentionally shut down, greatly reduces the drag that would occur with the blade pitch in any other position. On a single engine aircraft such as a motor glider, feathering the propeller when the engine is shut down results in a significant increase in gliding distance. On a multi-engine aircraft, feathering the propeller of a failed engine results in both a reduction in drag and a reduction in adverse yaw vastly improving the engine-out handling characteristics and the engine-out flight performance of the aircraft.

(SKYbrary - Feathering)
See also: What does feathering mean and how does it work technically?

Propellers get more effective the faster they spin. The limiting factor is the speed of the blade tip, which must remain subsonic. So a propeller of a given size will offer maximum thrust when its RPM gives a blade tip speed of around Mach 0.8.
Maximum thrust is needed especially for takeoff and for maximum speed. When the plane is stationary on the ground the prop spins wholly sideways, but when at speed it has to follow a much more angled spiral path through the oncoming air. These varying conditions can only be met by varying the blade pitch.

Effective and efficient are not the same. Propellers do become more effective the faster they spin (until the tips reach the critical Mach number), but they are most efficient at some optimal blade angle of attack (just like all airfoils). It is more efficient to reduce the prop RPM when less power is needed for cruise.

Point taken. I have edited my answer accordingly.

So there is another advantage - one can choose between max effectivity and max efficiency modes.

Reverse Pitch - Reverse Thrust
If the pitch angle is negative (reverse), then some meaningful amount of thrust is produced in the opposite direction than normal.  This can be useful to improve stopping distance when landing, and even taxi in reverse.
https://www.skybrary.aero/index.php/Reverse_Pitch#:~:text=When%20installed%2C%20reverse%20pitch%20is,of%20motion%20of%20the%20aircraft.
Zero Pitch - Idling
At somewhere around a pitch angle of zero, the net thrust becomes zero (forward or reverse).  At this point the aerodynamic load on the prop is minimized.  This removes unwanted thrust when we want to stand still on the runway, as well as minimizes fuel consumption during idle.

A transmission could do both of those, too.

Propeller efficiency with pitch, taken from Aerodynamics for Naval Aviators, H.H.Hurt, 1965
Fixed pitch props need to be designed for a specific flight speed, which would invariably be a compromise between takeoff and cruise performance. By varying the propeller pitch, we can run the propeller blades at different angles of attack, and we slow the blade tips down to keep their relative speed (sum of freestream and tangential speeds) constant. Thus, we can run the propeller at a similar efficiency through all flight speeds.
Running the blades at certain angles of attack is not as simple as it sounds, with different tangential speeds along the radius of the propeller, the freestream and tangential speeds will yield different flow angles (highest at the hub, least at the tip). To achieve the same angle of attack everywhere along the blade, we'll then need to twist the blade, but as we cannot change the twist in flight, we can optimize only for one flight condition, and be content with a compromise.

Propeller blade twist. Prop's feathered in this picture, prop turns counterclockwise from the pilot's perspective (this blade moves down).
Another aspect is related to the performance of the engine. Reciprocating (piston) engines only recently had electronic fuel injection systems, thus most aero engines are reliant on having a throttle that restricts airflow to the engine to adjust power. To run the engine at part-power with a throttle is quite inefficient, as the engine actively spends power to suck air through the throttle (This is why newer auto engines have variable valve timing and cylinder shut-off and whatnot, to adjust engine power without choking it). By running the engine at an RPM lower than where it produces maximum power, we can run the engine at a higher throttle setting (ideally full throttle) with the same power.

Porsche 930 dyno graph.
Increasing blade pitch reduces RPM and moves the power (red graph) to the left of the graph, meaning less power at full throttle.
Porsche 930's engine was used as an aero engine by Mooney, with a 0.442:1 reduction gear.


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Could a controller or airport in the US decide to use English-style landing clearances?
In the U.S., controllers are allowed to clear multiple planes to land on the same runway, with the assumption that the first plane to land will be out of the way by the time the second plane gets there. However, in England (and I think a lot of other places), you're not allowed to clear a plane for landing until it's the very next plane to land and there are no takeoffs or crossings ahead of it.
Could an American controller, or airport as a whole, just decide not to do sequenced clearances? That is, if they need to cross a plane in front of one that's landing, tell the landing plane "Cancel landing clearance but continue approach", then re-clear them to land once the runway is clear?

I don't see any reason why they couldn't, but I also don't see any reason why they would...

Are you talking about controlled or uncontrolled air ports? To be honest I don't believe that a controller in Britain will nor can do what you describe.  I think you misunderstand the  separation instructions that controllers give to pilots at a controlled airfield.

@TimothyTruckle I'm not asking about controllers in Brittan, I'm asking about controllers in the US.

@TimothyTruckle Given that HiddenWindshield is talking about controllers and landing clearances, I don't see how they could possibly be talking about uncontrolled airports.

According to this resource I misunderstood the meaning of "landing clearance"

The use of FAA's anticipated separation rule is theoretically optional, and there are indeed times when controllers don't use it. While not common, now and then one hears "continue approach" (or similar) instead of an early landing clearance, as is normal under ICAO rules. Ditto for the occasional canceled clearance.
However, anticipated separation exists for a reason, and a trainee who was unwilling or unable to use it when appropriate would not pass. OTOH, they also have to know when it's not appropriate and not use it at those times.


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Do you need to fill out the MedXPress form every time you renew your medical?
In the US, to get a 3rd-class medical certificate I know that you have to fill out this form on the FAA's website, get a confirmation number, then take that confirmation number to the medical examiner. My question is, do I have to do that every time? Or is the one confirmation number good forever?

Each time you renew you will need to complete the medical history questionnaire on the MedXPress website and obtain a fresh confirmation number.


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Does a single-engine-land license plus a multi-engine-sea license allow you to fly multi-engine-land and single-engine-sea?
I have a single-engine land PPL. I'm looking at getting training in a multi-engine seaplane. Does the fact that I already have a land airplane rating allow me to fly a multi-engine land plane?
(I did find this question, which seems to indicate the answer is "no", but I want to make sure.)


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Does the FAA have specific rules about whether craft in ground effect are actually "flying" or not?
From what I understand, the FAA doesn't consider anything in ground effect to be "flying", even if it's not actually touching the ground. This means, for instance, that the FAA wouldn't have jurisdiction over hovercraft, and it's what allows helicopters to hover-taxi around without having to contact the tower for takeoff clearance.
But I can't find any official rules stating this. I've found several assertions that it's true, but nothing official to back them up.
So, is there some rule in the FARs, or an official FAA publication, or something, that states this?

@757toga Well, there's this news article.

That's any interesting article but I question that these WIG "are already operating between mainland Italy and the west coast islands of Ponza and Ventotene". All I found was announcements from 2019 that they want to acquire them and that the service "could be up and running within a couple of years"...

@user2705196 The question isn't about whether these specific aircraft/boats/whatever are actually in service. The question is about whether "in ground effect" counts as "flying" from an FAA legal perspective.

I think the technical or legal consideration of whether or not a particular craft is "flying" is less relevant to the FAA than the altitude at which it operates.  A really tall cruise ship protrudes higher into navigable airspace than a hovercraft, should they regulate the hovercraft but not the ship?  You have to draw the line somewhere, and contact with the surface is largely irrelevant in this example.

@MichaelHall Do you have an official reference for that? Because that would be a perfect answer to my question if so.

I don't.  It kinda feels like a "can't prove a negative" type question.  (I don't know that they DON'T care about sea-skimmers, but I'm not aware of regulations that cover them, therefore...)  This reminds me of my answer to the question of flying over your own property without a license - the FAA isn't interested in "controlling" traffic in class G airspace, even if they have some jurisdiction.  Hmmm, maybe this is the basis of an anwer?

@MichaelHall Well, obviously if there aren't any such official regulations, then it would be proving a negative. But if there are, then surely someone knows what they are. (Just to be clear, I'd also be happy someone could point to something like an FAA Letter of Interpretation. It doesn't have to be something in the FARs.)

To save others the effort of looking: 14 CFR 1.1 does not define the terms “flight”, “taxi” or “ground effect”.  However, “aircraft” references “flight in the air”, which could be interpreted to exclude hovercraft and ground effect vehicles.

61.5 could be interpreted to exclude them as well.

I cannot attest to what the FAA considers "flight" per the title question, but I believe that their jurisdiction (as well as the means and level of control they exert on aircraft) is less dependent on a legalistic or technical definition of this term, and more about categories and classes of what it considers “aircraft” as well as where such craft actually operate.  Let's consider the two examples you offered:
Hovercraft -
Physically disconnected from the earth as they ride on a cushion air, they are technically "flying".  However, they are not classified as aircraft by the FAA and the physical space they operate in is the same realm as boats, cars, trains, and other surface vehicles.  The FAA would have no more interest or need to regulate their activities than they would other modes of transportation that operate outside the National Airspace System.
Helicopters -
While "air" taxiing, helicopters are also flying per the technical definition.  However, air taxi is necessary for helicopters equipped with skids because they don't have rolling wheels to maintain contact with the ground as they move along the surface of an airfield.  Since FAA controllers at a tower controlled airport wouldn't necessarily want them to simply takeoff and depart from ramp parking, some means is needed to needed to allow them to move to the runway or other designated takeoff and landing area.  As they move along a designated taxiway mere feet from the ground it makes more sense for this movement to be monitored and controlled by the same person controlling fixed wing aircraft, despite the fact that they are actually "flying".  (i.e. they aren't talking to a ground controller because they are not considered to be flying, it is because they are on a taxiway!) There is no logical reason to talk to a tower controller until they are ready to enter or depart the tower controlled air traffic pattern.
In summary, while the FAA recognizes rotorcraft as a category and class of aircraft in CFR 61.5, it does not consider hovercraft to be aircraft for regulatory purposes. While you would be free to log your experience driving a hovercraft it would not count as flight time in the eyes of the FAA.

Logically, I think you're right. But the law can, at times, have only a passing relationship with concepts like "logic". I'm specifically looking for official rules or rulings.

@HiddenWindshield you should read Oregon's distracted driving law; I think you will want to replace "only a passing" with "no."

@HiddenWindshield, see my latest edit. Hovercraft are not a category and class of aircraft. Excluded from FAA regulation by omission. That should shut the door in your question. (Thanks 757togo for pointing that out in your other comments…)

@MichaelHall "If a word is not defined in legislation, the ordinary meaning applies". This helicopter isn't technically airworthy. Could I legally hover-taxi it to the mechanic's shop without getting a Ferry Permit? Logically speaking, yes, because I'm not actually "flying" it, I'm taxiing it. But unless the word "flying" is legally defined, then the ordinary definition of "moving through the air under your own power" would be applied, which would make the answer "no".

@HiddenWindshield, if this the real reason for your original question then you should have just asked it this way.  I wouldn't have wasted all the effort discussing hovercraft...

@MichaelHall No, the real reason for my original question is BECAUSE I WAS CURIOUS. I know that hovercraft aren't aircraft. I know that taxiing helicopters aren't really flying. But where does the law say that? You assert that hovercraft are "not classified as aircraft by the FAA". As it happens, I agree with you. So do dozens of hits on Google (and those are just the ones I've checked). But where does the law say that?

@HiddenWindshield, "THE LAW" cannot detail every possible exclusion.  If something is not included, it is excluded.  I know that you know that, but if you need more affirmation than that I don't think you will be satisfied with any answer.  Good luck.  (maybe ask on Law SE?)

So besides the OP, who downvoted with zero feedback to improve the answer?

@MichaelHall Yes, I know that the law can't detail every possible contingency, but this one seems kind of important.

@HiddenWindshield, BTW, I think your question about whether a ferry permit is required to air taxi a non-airworthy helicopter is an EXCELLENT question!  I don't have an answer, and would love to see it posted as a separate question.  Otherwise my main critique is that the question is a bit broad.  We've beaten the hovercraft example to death, and as I see it the examples you gave are just too different to have a meaningful common answer.

@MichaelHall That's a good idea, I think I'll do that. Thanks!


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Do I need a Ferry Permit to hover-taxi a helicopter?
If a helicopter with skids is safe to fly, but not technically airworthy, do I need a Ferry Permit to hover-taxi it to the maintenance hanger on the other side of the airport? Are there any specific FARs that govern this?
(Based on the discussion in this question.)

@757toga If you had a fixed wing aircraft that wasn't "airworthy" by FAA standards but was entirely safe to taxi, could an appropriately rated & current pilot taxi it on the airport over to a maintenance facility? Would that qualify as "operating" it?

And do FAA regs even apply to the location? Standard common law says that a land owner also owns the airspace above the property.[with a bunch of logic contortions to let aircraft legally pass over without trespassing]  And since construction of up to 199.9 feet doesn't need to be registered with the FAA as an obstruction[a rule for cell towers] which you will be below, and you have the permission of the airport land owner to be there. There may be a legal case that you can hover taxi without normal flight requirements even if the FAA doesn't like it. This is not formal legal advice.

@MaxPower, the general rule is that if you're airborne and within the territory of the United States, the FAA has jurisdiction.

@Mark yes in general. But specifics do matter in law, such as in the case of staying within the private airspace of a property owner. (With the owner's permission.) Especially while at an altitude below normally accepted minimum altitude for general travel. Long standing case law precedent says "cuius est solum, eius est usque ad coelum et ad inferos" -- “whoever’s soil, it is theirs all the way to Heaven and all the way to Hell.”
Which is why stepping over a 4 way property corner diagonally, can get you cited for trespass even though your feet never touched the other two properties.

How heavy is this helicopter that you can't stick it on a flatbed and tow it to its destination?

@MaxPower The Supreme Court disagrees with you.  Take a look a United States v Causby https://supreme.justia.com/cases/federal/us/328/256/

@Adam The opinions of that case agree with my point. In fact they expressly say that use of the airspace below minimum safe altitude for en-route flight is an intrusion onto the land owner's property similar to conventional entry. Even the reduced altitude is for landing and taking off.


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Is there a specific reason Delta Airlines is allowed to have a call sign that's also code word in the NATO phonetic alphabet?
This is a followup to Is there any specific reason why Delta Air Lines doesn't have its own callsign? I was... "helpfully" informed that, um, actually, Delta Airlines does have its own callsign, it's "Delta".
So, let me try this again.
Having an airline callsign be the same as a letter code word in the NATO alphabet creates a huge potential for misunderstanding. Indeed, the controllers at Atlanta, Delta's headquarters, use "Dixie" for the letter "D", specifically to avoid ambiguity with the name of the airline. It seems to me that it would be much simpler to just assign the airline a different callsign. Is there any specific reason why this hasn't been done?

@ymb1, well, all in that article suggests that Delta should change their call-sign to avoid similarity with German aircraft using their tail numbers (which start with D).

I don't see how this is an issue at all. There's little chance of confusion for an aircraft whose callsign contains a "D" (November seven two two charlie delta) being confused, even with "Delta seven-twenty-two". If N722CD were a potential problem, I'd use his aircraft type rather than "November".


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Circa 1942 Republic of Ecuador - Can someone identify the aircraft in the background?
Circa 1942 Republic of Ecuador - Can someone identify the aircraft in the background?

Looks like a Fairchild PT-19...
https://en.wikipedia.org/wiki/Fairchild_PT-19

Thank you very much, XXavier!

@Francis If this answer was helpful, it would be good to upvote it. If you believe it answers your question, you can also accept it. You can learn more about how this site works in the [tour].

"PT-19" is not completely wrong, but to be more accurate, it is a Fairchild M-62A (the commercial equivalent of the PT-19).  The serial number ("5") on the fuselage confirms it as the last M-62A purchased.  The first PT-19s did not arrive in Ecuador until 1943.


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What's the difference between the Multi Engine Instrument Rating and Instrument Rating in Canada?
Are the requirements for the Multi Engine Instrument Rating the same as Instrument Rating in Canada or is there a separate exam for each of them?
Thanks!

Your IFR is either Group 1, 2 or 3.

All aircraft, including single, multi, multi centerline thrust (like a Skymaster).
Multi centerline thrust and singles.
Singles only.

For group 1 you have to do the ride in a conventional twin, which means you need a multi-engine rating first.  There is no written exam for the multi. It's just the training course and recommendation from the instructor for the check ride.
You can do a Group 3 IFR in a 172, but for the Group 1 you will still have to do a new check ride in a twin, so there is not much point in doing a Group 3 unless it's just to fly your own airplane IFR.
Just about everybody incorporates their multi engine training into the IFR training, does the Multi checkride at some interim point, then the IFR checkride in the same twin later on.
The main difference between the IFR checkride done in a single, vs one done in a twin, is the examiner gives you an engine failure drill to do on a missed approach when you do it in a twin. Otherwise they are about the same.

So if I have a commercial IFR rating and a multi engine rating, does that entitle me for a group 1 IFR? Thanks for the help!

If you had the multi-engine rating and then did the instrument rating flight test in a twin, as a Group 1 check ride, yes.  If you had an instrument rating BEFORE you did a multi, that would be a Group 3 instrument rating, and to obtain a Group 1 you would have to do a Group 1 check ride in a twin.  It's possible that upgrading from Group 3 to Group 1 they might only require you to do the engine failure/missed approach demonstration.   You'd have to ask an instructor.


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Can one use a calculator for the EASA part 66 exam?
For the first 3 modules you will need to do some calculations, is a calculator allowed?

Calculators are not allowed during examination. Therefore all calculations should be feasible without a calculator. Where a question involves calculations not feasible without a calculator, such as root 10, then the question should specify the approximate value of root 10
I found the answer on page 7 of this pdf.

Can you post the link to that “some site”? Definitely someone will benefit from it


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Why do modern fighter jets use 20mm guns?
Why do modern fighter jets use 20mm guns? Is the armor of a modern jet that thick? Does, for example, .50 (12.7mm) ammunition lack velocity to be effective?
I'm just curious on why is that 20mm is so widely used in air-to-air guns.

Welcome to SX! Please try to put a bit more effort into researching and formatting the questions. They are persistent, and should preferably be applicable to a broader audience. Keep the title simple and put your research into the post body. Unfortunately I couldn't understand your third question at all.

Remember that 20 mm canons were almost necessary even soon in the WWII. Only a minority of fighters kept .5 machine guns only and .3 machine guns only Spitfire I was too weak right from the beginning. To some extent that is due to armor that might not be used any longer, but  just a bullet without an explosive charge simply does not do enough damage when hitting a plane.

FWIW so as to not compare apples to oranges here, a .5 caliber barrel has an inner diameter of 12.7mm, and a 20mm cannon would translate into .78 caliber. Caliber being synonymous with inch in this context.

Last time I flew on a jet, there were no cannons at all -- was that 737 defective, or did the airline just cheap out on the options?

20mm is the smallest caliber that is practical to load with a bursting charge. Some fighter jets use 23mm, 27mm, or 30mm guns.
The reason 20mm has become one of the most common aircraft gun calibers is that it produces the lightest gun that meets the minimum requirements to be useful in air combat: enough rate of fire to hit a maneuvering jet, good enough ballistics for meaningful range, enough bursting charge capacity for meaningful damage.
The majority of modern combat jets do not carry armor, but they are built with considerable redundancy and internal subdivision. A bursting charge (explosive payload) is needed to do meaningful damage to a modern combat aircraft.
Smaller calibers can cause some damage that will require repairs, but not enough to rely on them to bring down a jet in combat. With missiles prioritized, most air forces want the lightest gun that's still effective.
Larger calibers have a cost in design. A large caliber rapid-firing gun will be much heavier, its propellant gases (muzzle exhaust) can cause flameouts in the engine, and firing will throw the aircraft's aim off center. A slower-firing one can be suitably light, but still has the same recoil force. Another issue with large calibers is collateral damage - 30mm+ rounds send fragments a couple hundred meters, far enough to damage friendly aircraft in air combat maneuvering.
The 20mm rounds used by combat aircraft have limited armor-piercing capability anyway (comparable to .50 BMG). Armored aircraft are rare, and some exceptions include CAS aircraft such as the A10 and to a lesser extent the Su-25 and the Su-34. This armor is mostly designed to protect them against ground fire, not enemy fighters.

There is, in fact, armor on multiple modern jets; an easy example is the A-10. I assume you meant to narrow that statement to fighters, but I think the answer can be improved in that direction.

You could also say that the 20mm Vulcan hits a sweet spot for a lot of areas at once; rate of fire and hence hit probability, ammo load and hence firing duration available, and so on, all maximized by using the smallest calibre with decent lethality.  Contrast it with the 37mm single barrel gun used on Soviet aircraft, where the hit probability was so low due to the low rate of fire, it negated most of its benefits.  Rate of fire is everything in air to air.  The German 30mm Mk108 was so effective because it has an unusually high rate of fire (600rpm IIRC) for such a large single barrel gun.

@AEhere True, I was addressing just the most stereotypical scenario of air combat. Edited to expand.

Therac: figured as much +1'd
@JohnK wasn't the 37mm used in conjunction with 20mm, as was usual in WW2 and early jets? Nowadays the Su-27 uses 30mm ammo with at over 1500rpm, still slow compared to the 6000rpm of the Vulcan.

When was did it became mainstream ?

@JohnK, your point is correct, but are you comparing WW2-era 37 mm guns with Vulcan? 37 mm hasn't really been used since early jet fighters (late 40s); contemporary aircraft use(d) 20, 23 or 30 mm.

No actually I was comparing the vulcan to the rivals at the time the vulcan came out, like the heavy single barrel cannon used on post war soviet a/c.   US pilots in Vietnam were apprehensive of the 37mm on Migs but realized quickly that its rate of fire was so slow the hit probability was tiny.

Could you also add something about the backwards thrust of larger caliber rounds? For instance, the A-10's GAU-8 has enough thrust that two would push the plane backwards. A bigger round means more recoil pushing back.

@Anoplexian Recoil is a factor, but pushing the plane back isn't a serious concern in aircraft design. Discharging the entire payload of the GAU-8 all at once will only cause the A-10 to lose 50-70 knots of speed - a slowdown, but not down from loiter to a stall, much less to going backwards. Discharging it in 3-4 very long bursts will reduce the speed loss to just 20 knots.

@Therac On the contrary, the aircraft was specifically designed around the gun, and the thrust is a huge issue with this gun and specifically the A-10. It's not just limited to the warthog either, the recoil force of MiG-27's GSh-6-30 is even larger at 60 kN compared to 45kN from the GAU-8 where the A-10's engines produce 40kN. Also, cannon-smoke for the air-intakes on the engine is also an issue for the A-10.

@Anoplexian The force and the vibration of the recoil, as well as the off-center effect of tilting the aircraft throwing the gun's aim off, and propellant gas ingestion, are simply much greater design concerns than direct thrust loss from recoil. If, say, the A-10 had 10 kN or 100 kN engines, it wouldn't directly affect its usable gun size. Being a larger tougher or smaller lighter-built aircraft would.  Anyway, I've elaborated on this in the answer.

If you want to know why you put a caliber usually between 20 and 30mm on a fighter plane (if you do any at all) you have to consider a few things.

Modern fighter planes can achieve speeds of more than 600 m/s. Even in maneuvering combat - which today is rather non existant - they still move at 200 m/s.
Engagement distances are therefore (turning circles etc. included) at least 500m to 1000m or far far more.
Weight on a plane is very much limited due to structural components, fuel and mission payload (which includes bombs and guided missiles).

With that in mind think about what a gun can achieve:
Typical weight for a gun system of calibres 30mm and higher is in the range of several tons. The GAU-8/A Avenger (the A-10 gun) has a (dry) weight of 1800kg. That is without ammunition, which also adds considerable weight. Thus it limits the payload for other ordnance available. Anything larger is fairly impractical due to weight considerations.
You are fighting at high velocities and at long ranges. You need a gun that has a high range of fire and a high projectile velocity. Thus anything below a .50 caliber gun has troubles considering the ranges involved (projectiles slow down considerably if fired at longer ranges and need to be fired in a higher arc).
Even with a high rate of fire (say 6000 rounds per minute) you end up with a space of 10 meters in between every round at the respective muzzle velocities (~ 1100 m/s). Your target is also moving at a considerable speed, thus even if you fire a long burst at a passing plane, it is unlikely to hit with more than a handful of rounds - if you hit at all.
Thus you want to maximize the effect you have, which means you need a projectile with a bursting charge. Otherwise it might just pass through the enemy plane without dealing real damage. Control surfaces, tanks etc. are all not so much a problem with just a small hole in it. This means that you need a bursting charge - which needs a larger projectile, thus anything below 20mm is almost impractical, because the charge would be too small.
This leaves us with guns in (usually) the 20mm - 30mm range.
Almost all fighter plane and fighter bomber guns are in that range. A few examples:

M61A1 Vulcan (US) - 20mm rotary cannon@6000 rounds per minute (the high rate of fire ensures more hits)
Mauser BK-27 (DE) - 27mm revolver cannon@1700 rounds per minute (larger projectile, thus greater bursting charge)
GSh-30-1 (RU) - 30mm cannon@1500-1800 rounds per minute (again larger projectile but less rounds)
GIAT30 (FR) - 30mm revolver cannon@2500 rounds per minute
GAU-8/A (US) - 30mm rotary cannon@3900 rounds per minute (really high weight for gun system and ammunition, but isn't meant for air to air combat anyways)

Technical correction: The GSh-30-1 is a rapid-fire single chamber recoil operated autocannon, not a revolver. The GSh design is exceptional in having the rate of fire of cannons twice its weight, but pays for it in being heat-limited. Also, one important factor with Gatlings is that the stated rate of fire is maximum, not average - much lower during the spin-up time. The GAU-8 stands alone in being a real gun for repeated prolonged firing, rather than a fighter weapon meant to only fire a few bursts.

You were probably thinking of GSh-6-30 (which is always worth mentioning; like GAU-8, its a peak gun in many respects). That's a 6-barrel revolver (with 5000+ rpm!) But then, see @Therac's comment. Its total live is less than MTBF (jams) of the Vulcan.

@Zeus You're both somewhat right. I mixed up the guns somehow, since GSh sadly has built a lot more 30mm cannons.

One should remember also "ballistic drop" because that greatly complicates shooting a moving target at distance. And fighter jets are already moving fast. Narrowing the "3D-ness" of where the round should end up makes it easier to be far more accurate, one imagines, and gives more time for other considerations a pilot may have...
Just going on the assumption a round is the same length, whether 20mm or 30mm, which does not seem to be so, resulting in ONLY the increased diameter of perhaps similarly weighted materials to be important, which does not seem to be so (but both "not so's" seem to be conservative), a 30mm round would weigh 2.25 times what a 20mm round weighs. That would mean, if the energies at firing were able to be equal, unless one chose to make them different, that the "velocity squared" term would be 2.25 times greater, or in other words, that velocity would be 1.5 times greater. Then, ignoring that a smaller round ought to confront less wind resistance and therefore maintain its velocity longer (in time) than a bigger round, the 20mm round will be faster at all points along its flight path so it will reach whatever target in 2/3 the time the bigger one will.
Remembering Galileo, both dropping due to gravity at the same rate, the shorter flight time will result in less drop, accentuated by the fact that drop accelerates as time increases. In fact, a small (small) amount of research indicates it might be about 4/7, call it 55% of the larger round's drop.
Given the conservatism and ignoring how independent choices might make some of it moot anyway, I'm comfortable moving on with the idea of the drop being 1/2. So your round might fall, say, 200 feet below the other aircraft instead of 400 feet. Both can be planned for and a gun can be mounted to make it look like a laser shot for a pilot (at a single, chosen range), but it seems targeting a target might be a LOT easier to be accurate about when allowing for the much smaller drop. Also, the greater the drop, the higher above the target one must aim and therefore the longer the parabolic path the round must take increasing the loss of velocity vs. the smaller round, and on and on...
Easier to at any given accuracy means quicker, less time spent on its mechanics, and so, the extra second here and there. Seems like those seconds could matter a lot on their own.
Penetrating power would strongly depend upon energy content at impact for any given material/geometry. The round that takes about 2/3 the time to "get there" seems, by the same small (small) amount of research, to have 4/3 the energy at impact making it likelier that it would penetrate so that the bursting would be internal where it would have far greater effect:  more ripping up flesh, electronics, flesh, fuel, flesh, hydraulics, flesh, radar profile, flesh... and less just messing up the paint.
Using the quick example in another Answer, I get a 36 foot separation between rounds. It seems to me that tracking the enemy so as to impact it with more than one round would be easier having a radically smaller drop to factor in. But I will say that it seems unlikely the designers planned for more than one round to hit as surely 2-3 guns firing at the same instant, a foot or two apart perhaps would be wonderful if they thought mor ethan one round was incredibly worthwhile and that just is not how the fighters are built so... Still, missing a few and tracking in on the enemy would seem easier as well with the smaller drop envelope.
Finally, the shorter time from firing to impact would have some smaller effects, like leaving less time for the enemy to maneuver out of the shot ("dodge")(realistically, more time for whatever instant thing he did to work, if it ever could... fewer of those extra seconds I mention above), like being able to take a meaningful shot on less time in frame (so taking a shot vs. "D*mn, too late to even take a shot"), like being able to fire in aid of a team member rather than worrying it might hit him, and other things that are even less quantifiable.

All of your calculations seem to be predicated on the idea that a 20mm round and a 30mm round would have the same kinetic energy at launch. But is that a reasonable assumption? A larger round would surely be equipped with a larger charge, leading to more KE at launch. One could presumably, in principle, arrange for the muzzle velocity of the two rounds to be equal, at which point all your calculations disappear, since the only difference would be the larger round having more air resistance, which you mention but don't include in your calculations.

@DavidRicherby Force exerted on a projectile through a barrel increased with the square of the diameter, mass with the cube. Thus to achieve the same muzzle velocity, pressure has to be higher on the larger projectile or the barrel longer. Higher pressure means thicker barrel with more mass. Longer barrel also means more mass. Both things are detriminal in terms of mountability on a platform already strapped for total mass. But yes, increasing propellant charge and barrel length can lead to higher muzzle velocity. Compare a 120mm tank shell @ 1700m/s+ on newer models.

@Adwaenyth Except this answer considers none of that. Also, it makes the simplifying assumption that the 30mm round is the same length as the 20mm one, which means that mass is also only increasing with the square of diameter so, under that assumption, it seems that the 30mm round would have exactly the same muzzle velocity as the 20mm round. So the calculations here don't seem realistic at all.

While this answer does state its hypotheses before calculating, I find those oversimplified and, to be honest, pointless... the muzzle velocity and shell geometry of the most common 20mm and 30mm A/C gun rounds are either publicly available or easy to approximate, with no need to resort to spherical cows on infinite planes.

authors: Vladimir F Героям слава


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What is the specific FAA regulation with regards to a pilot dropping jumpers into a solid cloud base?
What is the FAA regulation with regards to a pilot dropping jumpers into a solid cloud base?  I saw some parachute jumpers come through a solid cloud layer with a base at around 1300 AGL.  They landed maybe 1/2 mile from the airport on either side of a fairly busy four lane state road.  The cloud layer was solid 1600 AGL to 3000 AGL (we flew IFR and I recorded when we entered and when we got above the cloud blanket).  We were above this solid layer for 2 hours of our 3 hour flight.  Once home, I looked for the FAA regulation with regards to pilot's rules when dropping jumpers in such conditions but could not find any.  It is dangerous for the jumpers, the cars on the highway and for IFR pilots flying in the vicinity but but is it regulated by the FAA?


text:
When is the word "Over" required in radio transmissions?
From the AIM 4-2-3:

The terms initial contact or initial callup means the first radio call you make to a given facility or the first call to a different controller or FSS specialist within a facility. Use the following format:
[...]
(e) The word “Over” if required.

It is mentioned later than "Over" can be omitted in subsequent contacts if the transmission requires an obvious reply and there is no possibility of misunderstandings, but no other guidance is given for initial contact.
How do I determine if the word "Over" is required on initial contact?

If "over" is generally required, but there is an exception for certain subsequent contacts, wouldn't that imply that it is required in any initial contact?

@Someone On initial contact it already says "if required," it just doesn't clarify when it is required, so it's not clear to me.

It is neither required, nor necessary for brief, clear, and concise ATC to pilot communications on VHF and UHF when standard phraseology is used and the essential elements of a typical transmission are all present.
For example, "N123SM taxi Runway 16 via Alpha" is complete.  There's nothing else to say, nothing more expected.  As soon as the controller unkeys their mic it is obvious that the pilot should now respond.  The word "over" is simply unnecessary.
Where it becomes necessary, (and is expected) is when communication becomes lengthy, non-standard, or transmission quality is low.  A perfect example is a long range mid ocean phone patch via HF radio.  Reception may be weak and fuzzy, and you may have a lot more to say. You may even unkey the mic momentarily to take a quick breath before continuing on.  It is important that the receiver understand when you have finished so as to not step on you with a response before you have passed all the information along. This is when the term becomes useful.

"For example, "N123SM taxi Runway 16 via Alpha" is complete." How do we know that? What if it was intended to be "N123SM runway 16 taxi via Alpha, hold short of Charlie"?

@user71659, because if the controller stopped talking and you read it back without including the hold short instruction, they would repeat it and expect you to acknowledge. Alternately you could remain silent, and when they ask you can reply "but you didn't say 'over'".  Let us know how that goes...

AIM aside, and relying on decades of flight experience, I only use "over" to clearly signify the end of a long-winded transmission.  Usually those don't occur on ATC frequencies, but do occur on company and other frequencies.  For some, a common occurrence is when providing a detailed, therefore longer, PIREP.


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Any unique characteristics to identify aircraft engine?
I love to find out the engine manufacturer and make, used in all of the aircraft I fly.
Can anyone provide hints on how to identify an engine uniquely?

The CFM56 on a Boeing 737NG is easy. It has a flat bottom.
The GENx was at first easy to identify due to sawtooth pattern chevrons near the exhaust. But now the Trent 1000 too uses it.

Any easier ways to identify most popular engines?
Pratt and Whitney? Rolls Royce?

Answer is "There is NO HINTS on how to identify an engine uniquely". Why ? Just take the Boeing 777-200(ER) : Pratt & Withney, Rolls Royce and General Electric. I can spot the differences, but I have no words to explain you how I know, it's "something in the eye", GE is rounder, RR is more like an ellipse, and PW is somewhere inbetween. You have to do researches and memorize, just like how you make the difference between a 747-400D and a 747-300 with CF6-80 : learn the airlines operating them (that's cheating, but that's the way) How to have the eye ? spend hours on Airliners.net (or alike)

Some specific aircraft-engine combinations can be identified, but the engines themselves, no, because you don't see most of them—the nacelle is designed for each aircraft-engine combination. For example

The CFM56 on a Boeing 737NG is easy. It has a flat bottom.

the CFM56 on a Boeing 737NG is easy mainly because it is the only engine type that aircraft uses. And the engines from the same series when used on A320-family don't have flat bottom!
For A320-family, the CFM56 engines use separate exhausts, with shorter nacelle and hot section jet extending behind it, while the IAE V2500 ones use mixed exhaust with long nacelle, so you can tell them apart on those aircraft. But on NEO both engine types used have separate streams, and the only visible difference is the slight one in shape of the blades.
It is actually quite common for aircraft to only use one engine type, and using more than two types is rare, so most of the time if you can tell the aircraft, including variant, you should already know.

While in many cases it does come down to 1 or 2 engine types, I'm not sure I would call it "extremely rare" to have more. The following all have 3 types: 747 (except -8), 767-200/300, 777-200/200ER/300, A330

@fooot, ok, not extremely.


text:
Why isn't luggage that goes in the hold scanned as thoroughly as hand luggage?
I'm a regular international flyer, my hand luggage is always scanned, and despite my best efforts it often gets searched, either due to mistakes on my part, or security being extra cautious. On my most recent flight I estimate about 5-10% of my fellow passengers also had their hand luggage opened and searched by hand.
On the other hand, I have never had any issues with checked in luggage, I've never been pulled to one side and asked to explain anything, and as far as I can tell my luggage has never been searched (unless someone has picked the lock and then been very tidy in re-packing it)
So, my guess is that either 1) checked-in luggage isn't scanned as well as hand luggage, or 2) checked-in luggage is scanned by some super-hi-tech scanners that are infallible in identifying suspicious devices and never need checking by a person.
If the answer is option 1) then why doesn't checked-in luggage need such a thorough check? What's stopping a bad-guy putting a couple of kilos of something dodgy into his bag that brings down the aircraft over the Atlantic?
If the answer is option 2) then why can't the infallible super-hi-tech scanners be used on hand luggage too so no-one ever needs to be pulled over for a manual search?

A good bonus question could be: is it possible to get back the checked-in luggage by security staff to be searched in front of the passenger in case of doubt on what is in it?

I've had checked baggage searched manually, TSA usually leaves a note inside the bag.

The main reason why checked luggage is treated differently is that you cannot access it during the flight, therefore eliminating some of the security concerns (like e.g. a knife in checked baggage is not a risk). When your carry-on bag is hand-searched, it is most likely because something could not be seen properly or looked suspicious on the X-ray.
The checked baggage is still scanned though. The TSA writes:

TSA screens approximately 1.4 million checked bags for explosives and other dangerous items daily. Upon check in, your checked baggage will be provided to TSA for security screening. Once the screening process has completed, your airline will transport your checked baggage on your respective flight as well as deliver it to the baggage claim area. The majority of checked baggage is screened without the need for a physical bag search.
Inspection Notices: TSA may inspect your checked baggage during the screening process. If your property is physically inspected, TSA will place a notice of baggage inspection inside your bag. This is to inform you that an officer conducted an inspection of your property.

Note that the TSA will notify you with a notice in the bag when they opened it. Other countries might handle this differently and just open the bag without telling you.
Some more details of the screening process are given in this article (emphasis mine):

Soon after your suitcase vanishes behind the rubber curtain, it merges with the bags of other passengers -- in large airports, thousands an hour. After traveling up, down and around a vast network of high-speed conveyor belts, it arrives in a cavernous warehouse-like facility. Here, it will be scanned by Transportation Security Administration staff to make sure there's nothing in it that shouldn't be there, primarily explosives, illegal drugs or weapons. If they see any reason to open it, they will. Your bag might also be subjected to other checks, including sniffer dogs, X-ray and laser scanners and machinery designed to detect trace amounts of banned substances. If it passes muster, luggage handlers load your suitcase onto a wagon with the baggage of others on your flight. This wagon is driven onto the tarmac and its contents packed into the hold of the plane.

Us mere mortals are not entitled to know of the proceedings taking place behind the thick rubber curtains of the baggage conveyor belt, for reasons too obvious to list here. However, this Youtube video does show us the very infallible super-hi-tech scanners your question is referring to.
At least in Atlanta, it seems obvious every checked in luggage goes through one of these infallible super-hi-tech scanners, which, I bet, use x-ray scanning and chemical detectors to asses the contents of the luggage while they are inside the device. These devices will most likely be in use in most of the larger airports around the world
The size of the infallible super-hi-tech scanners is, as one can see, quite large, so fitting them into the checkout stand would not be practical. I also suspect these devices are ridiculously priced, so one does not want to purchase or lease any more of them than aboslutely necessary.
And... I bet (yes yes, I know betting, guessing and such is not encouraged here) if the infallible super-hi-tech scanners detect something out of the ordinary, the suspicious luggage will be manually inspected. So the handling of hand- and checked in luggage is not different in this respect.

Excessive sarcasm does not a good answer make.

No, I was not being sarcastic, sorry if it seems exessively so, I guess i deserved the minus one and accept it. The fact remains, that the machines used behind the rubber curtains are pretty much the same, probably way more sophisticated actually, as the ones use at the check in counter. The performance of these devices is not published, for obvious reasons, so it would require speculation to give such "info" as an answer. You can rest assured though, that when travelling via big airports, the stuff going into the cargo hold is very  thoroughly screened.


text:
How does a hybrid airship take-off?
A hybrid airship generates lift from a combination of lighter-than-air buoyancy, and aerodynamic lift.
Some figures suggest that up to 40% of the lift comes from aerodynamic lift (wikipedia). So how does a fully laden hybrid airship achieve vertical take-off? If the airship weights 100 tons, and it needs to get 40% of it's lift from aerodynamic lift (requiring forward movement) then that's 40 tonnes of lift that needs to be provided from somewhere whilst the aircraft is accelerating to flying speed. Where does this come from?
My guess is that a hybrid aircraft can only achieve full load when operating from a conventional airstrip, and that vertical take-off is only an option when operating at 60% load capacity. Is this correct, or have I missed something?

They can tilt their rotors a bit and do a STOL.  Much depends on weight.

The 40% dynamic lift fraction only applies to very small airships. Since area only grows with length squared and volume with length cubed, that fraction goes down for large ships. A "Graf Zeppelin" sized ship (87 tons useful lift) could maybe achieve 5% dynamic lift fraction.


text:
Why are hangars so high?
Or, to be more precise why was the Type-C hangar so tall? Built in the 1930s across many airfields in the UK, it was built with a higher roof (11 m) than any contemporary aircraft. (higher even than the tail of the worlds largest aircraft the Tupolev ANT-20)
Even by the end of WW2 it was still higher than any UK aircraft, even the American flying fortress could fit in (height wise)
So, why so tall? were they expecting future aircraft to be much higher? Was it also intended to house blimps? or is there some other reason?

Gantry cranes.  Buildings where people work on big machines usually have high ceilings, and often those ceilings leave space for gantry cranes.  The cranes may have been removed or never installed.
This is not specific to aircraft.
Spacecraft:

Trains:

Submarines:

Turbines:

It would be quite hard to perform an engine change on a Handley Page H.P.54 Harrow (1936) without a big crane.  Engines needed more frequent maintenance back then.

Video clip of a gantry crane used for A380 construction:  https://youtu.be/_-S2tzxYPD0?feature=shared&t=140  The ceilings are still quite high in modern aircraft facilities.

More gantry crane: https://commons.wikimedia.org/wiki/File:Airbus_A321-211Iberia_EC-IGK_(5567789129).jpg

Not just the crane itself, but as the photos show, the ability to do anything useful with the crane requires anything up to the aircraft's height again to move components above it.

@TobySpeight I think this drives the height the most. The crane itself doesn't take a huge amount of space, but if (in the extreme case) you want to lift one aircraft over the other, you need a very high clearance

Yeah, you also need to be able to move stuff on top of other stuff, where both stuffs are commonly very bulky.

There is a line in wikipedia which I think supports this: Cross beams affixed to the roof structure were capable of lifting a 6-tonne (6.6-ton) load.

11 meters isn't tall. You should see a modern hangar...

Note how much headroom there is between the tip of the tail and anything on the roof that could potentially damage it.
The aircraft doesn't always sit perfectly level on its gear, the roof can sag, the floor can be uneven, there's lights hanging from the roof, and you have people working on the aircraft. Even more headroom was desired in hangar design, because aircraft were getting larger every year, and it wasn't easy to predict this growth.
Most British bombers kept using twin tails, which result in a lower total height than a single tail. Yet, there were single-tail proposals, and had one won, the plane would've been taller. Twin tails stayed, so they lowered the roof eventually. Large single-tailed American bombers, like the B-29 and B-32, would've been too tall for the Type C hangar. So would the Britain' own Short Sunderland.
So, headroom was provided both for practicality and for future aircraft which might have been taller than they turned out to be.

Well  the Lancaster bomber was almost 7 meters from the ground, which doesn't leave huge amounts of clearance if you need to jack up the aircraft. Remember, these are maintenance hangars. Notice how every car workshop you go to has tall ceilings too!
The ANT 20 you mention was 10.6 meters - bearing in mind the height of the hangar isn't the same as the internal usable space, or indeed the door opening height, it seems it would be unlikely to fit at all, let alone be an excessive design
Your own link then goes on to say that as designs did normalise and we reached what seemed to be the reasonable limits for bomber design, that the hangar design changed and the roof dropped.

According to the OP's link (https://en.wikipedia.org/wiki/Type-C_hangar), the door height actually is 35 ft, while the "clear height" inside is 35 ft 4 in.  So they did think of future-proofing by making the doors about as high as the lowest points on the ceiling, and yeah Wikipedia does seem to be saying intentional future-proofing and flexibility was a consideration for RAF hangars.

Other design considerations for any industrial building, not just aircraft storage, maintenance or manufacturing, are lighting, ventilation, separation of incompatible processes & fire control.
Higher ceilings facilitate better indirect, shadow-free lighting from natural sources or extremely bright point sources, out of eyelines.
Airspace over work areas improves natural circulation of fresh air & dispersal of paint, solvent & cleaning vapours, hot work gases & smoke. It promotes a safer & less noisy work environment.
You do not want the carpentry power tools creating sawdust clouds rolling along the ceiling into the welding bay. You don't want the machinists having to go outside to turn around a 20 foot length of stock to avoid the spray painters because they can't end-over-end it.
Fire can spread faster & be harder to isolate & fight effectively in a constricted space. It is helpful to be able to use firehoses from a safe(r) distance while remaining inside the structure. The aircraft of the first half century of aviation were outrageously flammable. High ceilings ameliorated this while delaying smoke saturation to some degree.

Very good points

Hangers were originally enormously high, because they were first built to accommodate airships. Dumping an airship on the floor would wreck it: they did not have undercarriage (at least, not for the bow and stern), they were up to 900 feet long, all the important stuff was along the keel for stability, and the whole framework was designed to be in tension, not compression. The main circular frames were designed like spoked bicycle wheels.
The building are called hangars literally because the airship was hung from the roof, before the hydrogen was vented for safety, and maintenance could begin.

Welcome to Av.SE!

To be a pedant, hangar is the French word for barn.

@DavidMarshall  Rats! Hoist by my own firecracker!

The type C hangar is nowhere near big enough for an airship at that time.  Also, it seems the type C was built for the RAF, but I see no indications the RAF ever had airships.  Almost no hangars were designed with airships in mind.  For those that were, it is obvious from the shape.

They must be that high because its an C shaped construction, all C shaped halls are like this, the benefit is a roof that dont need supporting down to the ground inside the building, this gives you a fully open and large areal with no obstructions... The same goes for thoose who wonders why they C shape a tunnel roof. The fysics behind the C shape are well explained.

The "type-c" in the name doesn't refer to the shape of the hangar which is actually parallelepiped...

Sorry, U shape is more correct, or bow. But the answer to why they need to be so tall is still the same, the shape of the construction need to be like that because it equalizes the forces from the top down to the ground trough the "bow" shape into the sidewalls. its a self carrying roof structure with no need for supports to the ground level.

Understand, anyway all the hangars for example in the pictures of the other answers have a simple flat roof, no inverted-U shape

A related issue with regard to roof height and pitch has to do with snow accumulation.  A steeper roof will be incapable of holding as much snow as a shallower one.  In some climates, snow accumulation would not be a design consideration, but in other areas a steeper roof may be cheaper than a shallower one which could support the amount of snow that might accumulate.

Kinda vaguely right - airship hangers were often very high for their width and the outer layer described a catenary curve.  But that's specific to airships which were taller than their width.   And the lack of internal supporting posts is definitely a benefit for parking anything big and unweildy.


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Is the use of 1/8 NPT ports on pitot/static instruments derived from an FAA standard?
On almost every pitot-static instrument used in general aviation aircraft, a 1/8 female NPT port is provided to connect the instrument to the pitot-static system. I was told that is is a requirement from the FAA but I have been unable to find any such requirement listed in any TSO document. Is this actually a design requirement or just a commonly-used standard?

There are at least several certified products on the market that do not use this port style, and I'm not aware of any requirement for what port style to use. I'd guess it may be simply commonly used based on the age of the design.

@selectstriker2, can you give me some examples of the certified products that you are referring to?

This is a good question. TSO C10c for altimeters, for example, does not specify the pitot/static port hardware.